The Estelada (in Catalan, "starry"), a Catalan flag with a blue hoist triangle and white star in it, was designed around 1904 by Catalan nationalists who had returned from Cuba after independence proclaimed in 1902, taking the Cuban flag as model. The flag was adopted by Estat Catala (Catalan State), a party that proposed the creation of an independent Catalan state, whose flag would include a blue triangle (since the flag without triangle is considered the flag of the Catalan nation but not of the state).
The flag became so popular that it was one of the two flags used on 14 April 1931 in the proclamation of the Catalan Republic, a short-lived republic that was rescinded within one week under pressure from the Spanish Republican government. Esquerra Republicana de Catalunya (Catalonian Republican Left), the party that governed Catalonia between 1931 and 1939 adopted the flag, before merging with Estat Catala.

The Estelada is also used with a darker blue triangle. The white star is often pointing towards the upper hoist (photo). The design is obviously more suitable for flags hoisted vertically, as is frequently the case, but there are also many examples of this flag hoisted horizontally (photos).
The star is sometimes thicker, as seen, for instance, in 1998 on the headquarters of Esquerra Republicana de Catalunya in Reus).

Supporters of Juventud de Badalona, notorious in basket-ball and locally know as la Penya (the Friends' Club), designed a green and black estelada, with the club's colours. The flag, as seen on a photo, is more oblong than a typical estelada.

An estelada repainted into white and
blue, the colors of RCD Espanyol, a sports club from Barcelona, was
created by the NGO L'Associació per al desenvolupament de projectes solidaris al Sàhara (APSS - Association for the Development of
Solidarity Projects in Sahara), and is being sold at their website, the income being dedicated to aiding the diabetics from Western Sahara sheltered in the refugee camp
in Tindouf, Algeria (photo, photo). The flag was already promoted at the football matches, but also provoked an incident at the assembly of shareholders of the club.
[e-notícies, 19 November 2012; ForoCoches, 19 November 2012]

The estelada with a red star on a black triangle is, as its design clearly suggests it, the flag of Catalan Anarchists.
A flag with a five-pointing star pointing towards the hoist was
seen in Barcelona in November 2017 (photo). A presentation of the estelada and its
variations shows another photo of the same flag, with no information on time and place of making.
Another version of the flag (photo, Rojo y Negro website) is charged with an eight-pointed star. This flag was reportedly already used in the 1970-1980s, the eight points representing the eight Catalan Countries (Pàgina de les Esteladas).

A flag proposed for an independent Catalan Republic is white with a red St. George's cross and the estelada in canton. The width of the cross should be 1/9 of the flag's width. The flags, hwever, are not always manufactured accurately, the cross being visibly wider sometimes (according to a photo, up to 1/6 of the flag width).